Catalyst composition, process for its preparation and use thereof

ABSTRACT

Catalyst composition comprising titanium chemically bonded to a silica support, characterised in that the silica support is a shaped extrudate of silica powder. The catalyst composition is very useful in the epoxidation of olefins into alkylene oxides using organic hydroperoxides. The composition can be prepared by a process comprising the steps of: (a) extruding silica powder into an extrudate having a selected shape; (b) calcining the extrudate; (c) impregnating the extrudate with a titanium-containing impregnating agent; and (d) drying and calcining the impregnated extrudate.

[0001] The present inventions concerns an epoxidation catalystcomposition for the epoxidation of olefins into alkylene oxides. Theinvention also concerns a process for the preparation of such catalystcomposition and the use of the catalyst composition.

[0002] Catalyst compositions based on the titanium-containing compoundssupported on inorganic siliceous solid carriers for use in theepoxidation of olefins into alkylene oxides are well known in the art.Examples of such catalysts are, for instance, described in U.S. Pat. No.4,367,342 and EP-A-345856. U.S. Pat. No. 4,367,342 discloses the use ofinorganic oxygen compounds of silicon in chemical composition with atleast 0.1% by weight of an oxide or hydroxide of titanium, whileEP-A-345856 discloses a titania-on-silica heterogeneous catalyst whichis obtainable by impregnating a silicon compound with a stream ofgaseous titanium tetrachloride followed by calcinations and hydrolysissteps and optionally a silylation step.

[0003] More recently, titania-on-siliceous support type catalysts havealso been described in U.S. Pat. No. 6,011,162;EP-A-734764 andNf-A-1008686. These references also list suitable siliceous supportmaterials including silica-containing refractory oxides such assilica-alumina and silica-magnesia, highly crystalline materials such ashigh silica-zeolites, silica-containing molecular sieves and amorphoussilica. All these support materials have in common that they have aporous structure.

[0004] For instance, EP-A-734764 mentions silicates and silica, thelatter preferably being synthetic porous silica composed of amorphoussilica particles coagulated or bound to one another. Specific examplesmentioned are silica gel, precipitated silica, silica powders like fumedpyrogenic silicas and several crystalline alumino-silicates. InNL-A-1008686 synthetic porous silica composed of amorphous silicaparticles coagulated or bound to one another is mentioned. Specificexamples described are silica gel, precipitated silica, fumed pyrogenicsilicas and crystalline porous silicas such as high silica contentzeolites as exemplified by silicalite. Also the non-crystallinemolecular sieve material MCM-41 is mentioned as a suitable material. InU.S. Pat. No. 6,011,162 synthetic porous silicas consisting of particlesof amorphous silica flocculated or linked together so that they formrelatively dense, close-packed masses, are described as suitableinorganic siliceous materials. Specific examples mentioned are silicagel and precipitated silica. Also mentioned are synthetic silica powdersconsisting of particles of amorphous silica flocculated in open-packedaggregates, which are exemplified by fumed pyrogenic silicas. Anotherclass of suitable materials mentioned in U.S. Pat. No. 6,011,162 is theclass of refractory oxides such silica-alumina, silica-magnesia,silica-zirconia and the like. Furthermore, siliceous molecular sieveslike MCM-41, MCM-48 and M41S are mentioned.

[0005] However, all three references discussed in the previous paragrapheventually appoint silica gel as the preferred support material. Thisis, for instance, illustrated by the fact that in all working examplesgiven in these references a silica gel is used as the carrier material.

[0006] In general, silica gels contain three-dimensional networks ofaggregated silica particles of colloidal dimensions and are typicallyprepared by acidifying an aqueous sodium silicate solution to a pH ofless than 11 by combining it with a strong mineral acid. Theacidification causes the formation of monosilicilic acid (Si(OH)₄),which polymerizes into particles with internal siloxane linkages andexternal silanol groups. At a certain pH the polymer particlesaggregate, thereby forming chains and ultimately gel networks. Silicateconcentration, temperature, pH and the addition of coagulants affectgelling time and final gel characteristics such as density, strength,hardness, surface area and pore volume. The resulting hydrogel is washedfree of electrolytes, dried and activated. The drying procedure affectsthe gel characteristics. Once the dried silica gel particles areobtained their shape is fixed and their dimensions can only be reducedby temperature treatment.

[0007] Although the silica gel particles are an excellent material to beused as support material for titania-on-silica catalysts, there is stillroom for improvement. First of all, silica gel particles cannot beshaped into any desired form. Generally, silical gel particles arespherical, e.g. obtained by spray-drying the gel or by spraying the gelinto an immiscible liquid (emulsion polymerization). Alternatively,granular gel particles are used or, if being too large, are crushed intosmaller granules. If such gel particles are packed into a bed, theyconsequently do not have an optimum shape to minimise the pressure dropacross the catalyst bed when the bed is in operation. Especially withsmall gel particles the pressure drop poses a problem. On the otherhand, small catalyst particles are favourable as in that way theeffective diffusion length can be decreased and thus more active sitescan participate in the reaction, thereby resulting in reduced residencetimes of the feed. Such shorter residence time is favourable as it meansless secondary reactions and hence an improved selectivity. In practice,therefore, a balance is sought in terms of gel particle size between ahigh number of active sites and an acceptable pressure drop across thecatalyst bed.

[0008] A further problem with gel particles is their mechanicalstrength. Although the strength of these particles is acceptable, it isgenerally insufficient to allow re-use of spent catalyst particles.Therefore, an increased mechanical strength would be desirable.

[0009] As indicated herein before the pore structure and surfacecharacteristics of the gel particles are determined in the gelling anddrying stage. Consequently, the characteristics of a dried gel particle,i.e. the form in which these particles are normally commerciallyavailable, can no longer be modified.

[0010] It has now been found that the aforementioned shortcomings ofusing silica gel particles as catalyst support material can be overcomeby using a silica extrudate as support material. Namely, an extrudatecan be made in any desired shape, so that it is possible to use catalystparticles having a shape which optimally reduces the pressure drop whileat the same time maximising the number of active sites available.Furthermore, extrudates generally have a higher mechanical strength thangel particles. Finally, the pore structure and surface characteristicsof silica extrudates can be effectively steered by the extrusionprocedure and auxiliaries used therein. Consequently, it is possible tomodify the intrinsic properties of the commercially available silicapowder, which is used as the starting material for making theextrudates. U.S. Pat. No. 5,808,136 describes a catalyst for makingvinyl acetate monomer, which catalyst contains palladium, gold andalkali acetate as catalytically active components on a support ofsilicon dioxide, alumosilicate or aluminium oxide. Silica is used in theform of tablets (see Comparative Example 2 and DE-C-3803895 andDE-A-3912504). Moulding silica powder into tablets is a discontinuous,relatively complicated preparation route.

[0011] U.S. Pat. No. 6,008,389 discloses oxidation catalysts based ontitanium silicate extrudates having a zeolite structure, to whichsupport subsequently is applied from 0.01 to 30% by weight of one ormore noble metals selected from the group consisting of ruthenium,rhodium, palladium, osmium, iridium, platinum, rhenium, gold and silver.Contrary to the catalyst of the present invention, these catalysts arenot obtained from a silica support to which subsequently titanium ischemically bonded.

[0012] Accordingly, in a first aspect the present invention relates toan epoxidation catalyst composition comprising titanium chemicallybonded to a silica support comprising a shaped extrudate of silicapowder by impregnating the extrudate of silica powder with atitanium-containing impregnating agent.

[0013] The silica powder used can originate from different silicasources. It could, for instance, be derived from fumed silica orsilicalite. Suitably, however, the silica powder is a precipitatedsilica powder or a silica gel grinded into a powder. Such grinding canbe performed by any suitable grinding means known in the art. Ingeneral, the silica powder particles will have an average particle sizeof 1 to 100 μm, although larger particles could also be used. The silicapowder preferably contains at least 99 wt % of silica. More preferably,the silica powder consists of silica.

[0014] Precipitated silica powder is composed of aggregates of silicaparticles of colloidal size that have not become linked in a massive gelnetwork during the preparation, but instead have coagulated intodistinct solid particles. Precipitated silica is typically prepared byprecipitation from a solution with a high sodium silicate concentrationunder certain specific pH conditions and using certain specificcoagulants. Alternatively, precipitated silica can be prepared by addingaqueous ammonium hydroxide to ethyl silicate in alcohol. In general,ultimate and aggregate particle size in precipitated silica preparedfrom an aqueous sodium silicate solution can be varied by reinforcementand control of suspension pH, temperature and salt content. Theseprocedures are known in the art. Suitable precipitated silica powdersconsist for at least 90 wt %, preferably for at least 95 wt % and morepreferably for 98 wt % or more of silicium dioxide. The powderspreferably have a specific surface area (BET) of from 200 to 800 m²/gand an average particle size of 1 to 100 μm. Such powders arecommercially available from several suppliers like Degussa andCrossfield.

[0015] The titanium present in the catalyst composition of the inventionsuitably is present in the form of titanium oxide or titanium hydroxide,more suitably titanium oxide. It is believed that the titanium is bondedvia one, two or three oxygen atoms to respectively one, two or threesilicon atoms which form part of the silica network. This is, forinstance, described in EP-A-345856.

[0016] The amount of titanium (as metallic titanium) will normally be inthe range of from 0.1 to 10% by weight, suitably 1 to 5% by weight,based on total weight of the catalyst. Preferably, titanium or atitanium compound, such as a salt or an oxide, is the only metal and/ormetal compound present.

[0017] The catalyst composition is suitably obtained by impregnating theextrudates of silica powder with a titanium-containing impregnatingagent. It was found particularly advantageous to calcine theseextrudates at a temperature in the range of from 400 to 1000° C.,preferably 450 to 800° C. and more preferably 500 to 700° C., prior tothe impregnation. Further details of the preparation process of thecatalyst composition of the present invention will be discussed hereinafter.

[0018] The final extrudates of silica powder, i.e. the extrudates whichare actually being impregnated, suitably have a surface area (asdetermined by BET method ISO 9277:1995(E)) in the range of from 100 to1000 m²/g, preferably from 150 to 700 m²/g and more preferably 200 to500 m²/g, a pore volume (as determined by mercury intrusion) in therange of from 0.5 to 2.5 ml/g, preferably 0.7 to 2.0 ml/g and morepreferably 0.8 to 1.5 ml/g, and a pore diameter as determined by mercuryintrusion in the range of from 3 to 40 nm, preferably 4 to 30 nm andmore preferably 4 to 20 nm.

[0019] One of the advantages of using extrudates is that their shape caneasily be varied. For instance, the shape of the extrudate ofprecipitated silica powder can suitably be selected from a sphere, atrilobe, a quadrulobe, a ring, a massive cylinder and a hollow cylinderand the average particle size of the extrudate ranges from 0.5 to 10 mm.When using a spherically shaped extrudate, the wet extrudate is firstspheronised in a suitable spheronising device before calcination. Theway in which the particle size is defined varies with the actual shapeof the particle, but the size given refers to the size definitionsnormally used. For instance, for spheres the average particle sizerefers to the diameter of the sphere, for rings to the outer diameter ofthe ring. For cylinders, it refers to the diameter of the circularcross-section of the cylinder and for tri- and quadrulobes to thedistance between the tangents of two opposite lobes. In case of particleshapes having a length-component, the length/diameter ratio willnormally be in the range of 1 to 5.

[0020] However, shapes other than those mentioned and sizes outside therange indicated may also be used.

[0021] In a second aspect the present invention relates to a process forthe preparation of a heterogeneous catalyst suitable for the epoxidationof olefins into alkylene oxides, which process comprises the steps of:

[0022] (a) extruding silica powder into an extrudate having a selectedshape;

[0023] (b) calcining the extrudate;

[0024] (c) impregnating the extrudate with a titanium-containingimpregnating agent; and

[0025] (d) drying and calcining the impregnated extrudate.

[0026] In step (a) the silica powder is extruded into an extrudate. Thiscan be performed by conventional extrusion methods and techniques knownin the art, such as e.g. disclosed in EP-A-309048. Typically anextrusion mixture is prepared from the solids (silica powder andoptionally binder), water and extrusion aids by mixing and kneading theingredients into a shapable dough and passing this dough into theextruder.

[0027] In addition to the silica powder a binder material may be used.Suitable binder materials include inorganic oxides like silica,magnesia, titania, alumina, zirconia and silica-alumina, of which silicais preferred. The weight ratio of binder to silica powder material mayvary from 0:100 to 90:10.For the purpose of the present invention thesilica powder is suitably extruded without additional binder material.However, if used, it is preferred to use the binder material in a weightratio of binder to silica powder material of from 10:90 to 50:50.

[0028] Beside the silica powder, optional binder and water the extrusionpaste will normally also comprise extrusion aids to improve the flowproperties. Extrusion aids are known in the art and may includeflocculation agents, which normally are polyelectrolytes. Furtherextrusion aids include, for instance, ammonia and ammonia-releasingcompounds like ammonium hydroxide, aliphatic mono-carboxylic acids,polyvinyl pyridine, and sulfoxonium, sulfonium, phosphonium and iodoniumcompounds, alkylated aromatic compounds, acyclic monocarboxylic acids,fatty acids, sulfonated aromatic compounds, alcohol sulfates, etheralcohol sulfates, sulfated fats and oils, phosphonic acid salts,polyoxyethylene alkylphenols, polyoxyethylene alcohols, alkanolamines(e.g. monoethanolamine), polyoxyethylene alkylamines, polyoxyethylenealkylamides, polyacrylamides, polyacryl amines, polyols, polyvinylalcohols, acetylenic glycols and graphite. Burnout materials may also beused to increase the porosity of the final extrudate. Examples ofburnout materials are polyethylene oxide, methyl-cellulose,ethylcellulose, latex, starch, nut shells or flour, polyethylene or anyof the polymeric microspheres or microwaxes.

[0029] The preparation of a shapable dough from the solids (silicapowder and optionally binder), water and extrusion aids can be performedby those methods known in the art for preparing extrudable doughs, suchas for instance disclosed in EP-A-309048.Such dough typically has apaste-like appearance. It is within the normal skills of those skilledin the art to optimise the mixing/kneading procedure and to select theappropriate extrusion equipment, to select the appropriate amounts ofthe ingredients to obtain an extrudable dough, to select the timing ofadding the various ingredients and to select the most appropriateextrusion conditions. Typically, the extrudable dough will have a solidscontent of 23-60% by weight, suitably 27 to 55% by weight. The extrusionaids are typically used in an amount of from 0.5 to 20% by weight,suitably 2 to 15% by weight, on the total solids content of the mixture.The remainder up to 100% by weight is water.

[0030] After extrusion the extrudates are calcined in step (b).Calcination is typically effected at a temperature of from 400 to 1000°C., preferably from 450 to 800° C., more preferably from 500 to 700° C.Calcination time may vary within wide limits and may range from 15minutes to 48 hours. Suitably, the calcination time will be between 1and 4 hours. The calcination may be preceded by a separate drying step,but this is not required. If applied, such drying step will typically beeffected at a temperature up to 300° C., more suitably up to 250° C. Theperiod for drying may vary, but will usually up to 5 hours, moresuitably from 30 minutes to 3 hours. The drying may be integrated withthe subsequent calcination or, as indicated before, be completelydispensed with.

[0031] The calcined extrudate thus obtained may subsequently be treatedwith water or steam before it is impregnated in step (c). Suchhydrolysis treatment may, for instance, involve a pore impregnationtreatment with water by soaking or immersing the calcined extrudate inwater or steaming the extrudate, e.g. with low pressure steam of120-180° C. Alternatively, the hydrolysis treatment may comprise awashing treatment using an aqueous solution of a mineral acid, anaqueous solution of an ammonium salt or a combination thereof. Withoutwishing to be bound by any particular theory it is believed that thehydrolysis treatment is beneficial for re-hyroxylating the silicasurface, i.e. restoring any silanol groups on the surface of the silicaextrudate which may have been destroyed in the preceding calcinationstep. Silanol groups, namely, are important for the titanium-containingimpregnating agent to react with the silica surface so that it ischemically bonded thereto.

[0032] Depending on which hydrolysis treatment is used the conditions atwhich this treatment is carried out may vary. A water soaking step isnormally carried out at ambient temperature, while a steaming step atthis stage of the process is suitably carried out at temperatures thattypically range from 120 to 180° C.

[0033] As indicated, the hydrolysis may also involve a washing treatmentusing an aqueous solution of a mineral acid, an aqueous solution of anammonium salt or a combination thereof. Suitable mineral acids in thisconnection include hydrochloric acid, sulphuric acid, phosphoric acidand the like. Particularly preferred washing liquids are aqueoussolutions of hydrochloric acid or sulphuric acid. Other washing liquidswhich can be used include aqueous solutions of an ammonium salt. Suchammonium salts also include tetramethyl ammonium salts. Examples ofsuitable ammonium salts, then, include ammonium or tetramethyl ammoniumhydroxide, nitrate, acetate, citrate, carbonate, chloride and sulphate.Of these, ammonium acetate is particularly preferred. Concentrations ofthe mineral acids or ammonium salts in water are not particularlycritical and will normally range from 0.01 M to 5 M.

[0034] Particularly if the hydrolysis treatment involves washing with amineral acid solution and/or an ammonium salt solution, an optionaladditional washing step may be applied: washing with water, preferablywith distilled, demineralised or deionised water. If applied, this waterwash step may be repeated one or more times. Suitably, the water washstep may be carried out one to six times.

[0035] The optional hydrolysis step may be followed by a drying step.Drying may take place in conventional ways known in the art and for thepurpose of the present invention it was found particularly suitable toperform the drying in an oxygen-containing atmosphere, suitably air, ata temperature of from 70 to 400° C., more suitably from 110 to 300° C.Alternatively, the drying may take place in an atmosphere other thanair, e.g. in a nitrogen atmosphere. Drying time will normally be between15 minutes and 5 hours.

[0036] In step (c) the calcined and optionally hydrolysed (and dried)extrudate is impregnated with a titanium-containing impregnating agent,while in step (d) the impregnated catalyst is dried and calcined. Thedrying in step (d) may be carried out in the same way as described abovein relation to drying of a hydrolysed extrudate. Calcination in step (d)is suitably performed at a temperature of from 400 to 1000° C.,preferably 500 to 800° C. Again, calcination time may vary within broadlimits and the same ranges apply as indicated hereinbefore in respect ofcalcination time of the extrudate in step (b).

[0037] The impregnating agent used in step (c) may be either a liquid ora vapour. If a liquid impregnating agent is used, an additional dryingstep may be included between steps (c) and (d) to remove the solventused in the impregnating solution. Examples of suitable liquidimpregnating agents are known in the art and include solutions oftitanium tetrahalide, such as titanium tetrachloride or titaniumtetrafluoride, in an organic solvent, such as alkanes (e.g. hexane),aromatic compounds (e.g. toluene), alcohols (e.g. methanol, ethanol) orethers. Other examples include organic titanium complexes such astetra(isopropyl) titanate, tetra (n-butyl) titanate, tetrakis(trimethylsily) titanate and di(acetoacetyl)di(isopropyl) titanate, thelatter being for instance described in JP-A-11/228553. Wet impregnationmethods are also well known in the are and in principle any suitable wetimpregnation technique may be used. Examples of such techniques aredisclosed in GS-1,332,527;EP-A-734764;WO-98/50374 and U.S. Pat. No.6,011,162.

[0038] In a preferred embodiment, however, a gaseous titanium-containingimpregnating agent is used. A gaseous titanium tetxahalide and inparticular gaseous titanium tetrachloride, optionally in conjunctionwith an inert carrier gas like nitrogen or argon, is very useful in thisrespect. A method using gaseous titanium tetra-chloride as impregnatingagent, followed by calcination, hydrolysis and optionally silyation isdescribed in EF-A-345856.This process is very suitable for the purposeof the present invention.

[0039] Accordingly, the present invention also relates to a process forthe preparation of a heterogeneous catalyst composition for theepoxidation of olefins into alkylene oxides, which process comprises inaddition to steps (a) to (d) mentioned hereinbefore:

[0040] (e) hydrolysing the calcined material obtained from step (d), and

[0041] (f) optionally silylating the product from step (e).

[0042] Further details regarding steps (c) through (f) for gas phaseimpregnation can be found in FP-A-34556,which is incorporated herein byreference. For instance, gas phase impregnation may be carried out usingan inert carrier gas at temperatures which suitably are higher than 130°C., more suitably between 150 and 250° C. Hydrolysis step (e) may becarried out by ways known in the art and examples of suitable hydrolysistreatments are described hereinbefore in relation to hydrolyzing theextrudate after step (b) and prior to step (c). However, when steam isapplied temperature conditions typically are somewhat more severe thanin the optional earlier hydrolysis step between steps (b) and (c).Accordingly, hydrolysis step (e) is suitably effected with steam at atemperature in the range of from 150 to 400° C.

[0043] Silylation step (f) can be carried out by ways known in the art,for instance by contacting the product of step (e) with a suitablesilylating agent at a temperature between 100 and 300° C. Suitablesilylating agents include organosilanes like tetra-substituted silaneswith C1-C3 hydrocarbyl substituents. A very suitable silylating agent ishexamethyldisilazane. Examples of suitable silylating methods andsilylating agents are, for instance, described in U.S. Pat. No.3,829,392 and U.S. Pat. No. 3,923,843 which are referred to in U.S. Pat.No. 6,011,162,and in EP-A-734764.

[0044] It is well known in the art to produce alkylene oxides, such aspropylene oxide, by epoxidation of the corresponding olefin using anactive oxygen species such as hydrogen peroxide or an organichydroperoxide as the source of oxygen. For instance, a commonly knownmethod for manufacturing propylene oxide is the co-production ofpropylene oxide and styrene starting from ethylbenzene. In general suchprocess involves the steps of (i) reacting ethylbenzene with oxygen orair to form ethylbenzene hydroperoxide, (ii) reacting the ethyl-benzenehydroperoxide thus obtained with propene in the presence of anepoxidation catalyst to yield propylene oxide and 1-phenyl ethanol, and(iii) dehydrating the 1-phenyl ethanol into styrene using a suitabledehydration catalyst.

[0045] Another method for producing propylene oxide is the co-productionof propylene oxide and methyl tert-butyl ether (MTBE) starting fromisobutane and propene. This process is well known in the art andinvolves similar reaction steps as the styrene/propylene oxideproduction process described in the previous paragraph. In theepoxidation step tert-butyl hydroperoxide is reacted with propeneforming propylene oxide and tert-butanol. Tert-butanol is subsequentlyetherified into MTBE.

[0046] The heterogeneous catalyst composition of the first aspect of thepresent invention and as obtainable by the process of the second aspectof the present invention is very suitable as an epoxidation catalyst forpromoting the epoxidation of alkenes into the corresponding alkyleneoxide. Accordingly, in a third aspect the present invention relates to aprocess for the preparation of an alkylene oxide by reacting an olefinwith an active oxygen species in the presence of the heterogeneouscatalyst of the first aspect of the present invention. Suitably, theactive oxygen species is an organic hydroperoxide, such as tert-butylhydroperoxide and ethylbenzene hydroperoxide, the latter being preferredfor the purpose of the present invention.

[0047] It was surprisingly found that the heterogeneous catalyst of thepresent invention resulted in an excellent propylene oxide selectivity,i.e. the mole percentage of propene that is converted into propyleneoxide, and/or excellent activity when compared to a heterogeneouscatalyst based on a silica gel carrier.

[0048] The conditions under which the epoxidation reaction is carriedout are those conventionally applied in propene epoxidation reactionswith ethylbenzene hydroperoxide. Typical reaction conditions includetemperatures of 50 to 140° C., suitably 75 to 125° C., and pressures upto 80 bar with the reaction medium being in the liquid phase.

[0049] The invention is further illustrated by the following exampleswithout limiting the scope of the invention to these particularembodiments.

[0050] In the examples the following ingredients are used: HP321:particulate silica powder ex Crossfield SIPERNAT 50: particulate silicapowder ex Degussa (SIPERNAT is a trademark) MEOA: mono-ethanol amineSUPERFLOC N100: flocculation agent (SUPERFLOC is a trademark) NALCO7879: flocculation agent (NALCO is a trademark) PVA: polyvinylalcohol

EXAMPLE 1

[0051] In this example silica extrudates are prepared and theseextrudates are subsequently used to prepare catalyst particles. Theingredients used in their respective amounts were: SIPERNAT 50 (g) 2867NALCO 7879 (g) 100 MEOA (g) 125 Water (g) 5550 Additional Water (g) 250

[0052] All silica powder was charged into a mixer/muller and the mixingand kneading was started. After 1 minute a solution of MEOA in water wasadded. After 35, 50 and 60 minutes respectively 125 grams, 50 grams and75 grams of additional water were added. After 90 minutes theflocculation agent NALCO 7879 was added. After 95 minutes the mixing andkneading stopped. The resulting plastic pieces were subsequentlyextruded.

[0053] Extrusion took place in a 2.25 inch Bonnot extruder provided witha dieplate of a 1.3 mm trilobe. The screw speed in the extruder was 20rpm (rotations per minute).

[0054] The extrudates thus obtained were dried for 2 hours at 120° C.and subsequently the temperature was raised to 800° C. over a period of3 hours, at which temperature the extrudates were maintained for 2hours. After cooling the extrudate strings were broken and sievedresulting in trilobes with a diameter of 1.3 mm and a length/diameterratio of about 3.

[0055] The extrudates were subsequently soaked in water for 1 hour,filtered and dried for 2 hours at 120° C. The properties of theextrudates are listed in Table 1.

[0056] 75 grams of the extrudates were loaded into a quartz reactor tubeand heated to 260° C. under a nitrogen flow of 73 Nl/h. The extrudateswere kept at 260° C. for 2 hours. Hereafter the extrudates were cooledto 195° C. and 15 grams gaseous titanium tetrachloride (TiCl₄) waspassed through the bed of extrudates over a period of 70 minutes while anitrogen flow of 5 Nl/h was maintained as well. After termination of theTiCl₄ distillation, dry nitrogen was led over the silica bed for 2 hoursat 195° C. and at a rate of 5 Nl/h. The impregnated silica was thenheated in a nitrogen atmosphere to 600° C. (at a rate of 50° C./h) andcalcined for 6 hours at 600° C. The calcined silica/titania catalyst wascooled to 325° C., while the nitrogen flow was increased to 10 Nl/h.Then steam was added to the nitrogen circulating over the catalyst.Steam treatment was thus carried out by passing steam over the catalystbed at a rate of 4 g/h for two hours at 325° C. The reactor wassubsequently cooled to 200° C. in a stream of dry nitrogen.Hexamethyldisilazane was then passed over the catalyst bed at a rate of18 g/h for two hours using dry nitrogen as a carrier gas (at a rate of 5Nl/h). An exotherm of 30° C. was observed, indicating a reaction ofhexamethyldisilazane with hydroxyl groups in the catalyst. Excess ofhexamethyl-disilazane was stripped with nitrogen (75 Nl/h).

[0057] The catalyst particles thus obtained are further referred to asCatalyst 1.

EXAMPLE 2

[0058] Example 1 was repeated except that the wet extrudate was passedinto a spheroniser after passing through a 0.8 mm cylindrical dieplate.In the spheroniser spheres having a diameter of 1.4 mm were formed.These spheres were subsequently dried, calcined, impregnated withgaseous TiCl₄, hydrolysed and silylated as described in Example 1(Catalyst 2).

EXAMPLE 3

[0059] The ingredients used in their respective amounts to prepare thesilica extrudates were: HP321 (g) 169 NALCO 7879 (g) 5 SUPERFLOC N100(g) 4.5 MEOA (g) 6 PVA, 5% in water (g) 61 Water (g) 256

[0060] All silica powder was charged into a mixer/muller and the mixingand kneading was started. After 3 minutes SUPERFLOC was added and after13 minutes a solution of MEOA and PVA in water was added. After 40minutes of mixing and kneading the powdery mix changed into a mixconsisting of plastic pieces and to these plastic pieces NALCO wasadded. After 44 minutes an extrudable mixture was discharged.

[0061] Extrusion took place in a 1 inch Bonnot extruder provided with adieplate of a 0.8 mm trilobe. The screw speed in the extruder was 30rpm.

[0062] The extrudates thus obtained were dried for 2 hours at 120° C.and subsequently the temperature was raised to 550° C. over a period of2 hours, at which temperature the extrudates were maintained for 2hours. After cooling the extrudate strings were broken and sievedresulting in trilobes with a diameter of 0.8 mm and a length/diameter 5ratio of about 3.

[0063] The extrudates were subsequently soaked in water for 1 hour,filtered and dried for 16 hours at 120° C. The properties of theextrudates are listed in Table 1.

[0064] The extrudates thus obtained were impregnated with gaseous TiCl₄,hydrolysed and silylated as described in Example 1 resulting intitania-on-silica catalyst particles (Catalyst 3). TABLE 1 ExtrudateProperties Example 1 Example 2 Example 3 Flat Plate Strength* (N/cm) 5558 71 BET surface** (m²/g) 245 231 364 Hg surface area (m²/g) 263 257411 Pore volume*** (ml/g) 1.27 1.21 1.06

EXAMPLE 4

[0065] Catalyst 1, Catalyst 2 and Catalyst 3 were used as catalyst inepoxidation experiments to convert ethyl-benzene hydroperoxide withpropene into propylene oxide and 1-phenyl ethanol.

[0066] The epoxidation experiments were carried out in a continuousepoxidation bench scale unit containing two vessels on automatic weightbalances containing respectively the EBHP and alkene feed streams, twohigh pressure pumps, a fixed bed reactor, a third pump for pumping arecycle stream over the reactor, means to maintain the reactorcontinuously at temperatures between 60 and 120° C., a stripper toremove light boiling components like propene, a cooler and a vessel forreceiving the product.

[0067] The feeds were supplied to the reactor via the two high pressurepumps and mixed together before entering the reactor. The reactor wasoperated liquid full at 40 bara pressure. A large recycle stream wasmaintained over the reactor to have isothermal operation of the reactorbed and to ensure that the catalyst to be re-activated is contacted withepoxidation reaction product. The feed of propene and a 35 wt % EBHPsolution in ethylbenzene was mixed with the recycle stream prior tointroduction into the reactor.

[0068] A compositional analysis of the reaction mixture was carried outby means of Super Critical Fluid Chromatography (SFC).

[0069] The following process conditions were maintained: throughput EBHPsolution: 30 grams/hour throughput propene: 18 grams/hour recycle flow:2.5 kg/hour.

[0070] The activity of the catalyst is expressed as “K85” indicating thereaction rate constant in kg² of liquid per kg of catalyst per mole perhour (kg²/(kg*mole*h)) normalised at 85° C. assuming that first orderreaction kinetics apply in EBHP and in propene. The K85 is determined asthe mean K85 over 300 hours of operation at 90° C.

[0071] The selectivity of the catalyst is calculated as the meanselectivity to propene over a period of 300 hours at 90° C.

[0072] The results are indicated in Table 2. TABLE 2 Epoxidation resultsCatalyst 1 Catalyst 2 Catalyst 3 K85 (kg²/(kg*mole*h)) 18.8 20.0 34.9Selectivity (wt %) 95.0 95.2 95.3

COMPARATIVE EXAMPLE 1

[0073] Example 1 was repeated except that the extrusion part was notcarried out. Instead, 75 grams of a commercial silicagel (G57 ex Grace)was loaded into the quartz reactor tube. The resulting catalyst was usedas an epoxidation catalyst according to the procedure described inexample 4,giving a K85 of 14.0 kg²/kg*mole*h and a selectivity of 90.4wt %.

1. Epoxidation catalyst composition comprising titanium chemicallybonded to a silica support comprising a shaped extrudate of silicapowder by impregnating the extrudate of silica powder with a titaniumcontaining impregnating agent.
 2. Catalyst composition as claimed inclaim 1, wherein the titanium is present as titanium oxide or titaniumhydroxide.
 3. Catalyst as claimed in claims 1 or 2, wherein the silicapowder is a precipitated silica powder or a silica gel grinded into apowder.
 4. Catalyst composition as claimed in any one of claims 1-3,wherein the extrudate of silica powder has a surface area in the rangeof from 100 to 1000 m^(2/)g, a pore volume in the range of from 0.5 to2.5 ml/g and an average pore diameter as determined by mercury intrusionin the range of from 3 to 40 nm.
 5. Catalyst composition as claimed inany one of claims 1-4, wherein the shape of the extrudate of silicapowder is selected from a sphere, a trilobe, a quadrulobe, a ring, amassive cylinder and a hollow cylinder and the average particle size ofthe extrudate ranges from 0.5 to 10 mm.
 6. Process for the preparationof a heterogeneous epoxidation catalyst composition for the epoxidationof olefins into alkylene oxides, which process comprises the steps of:(a) extruding silica powder into an extrudate having a selected shape;(b) calcining the extrudate; (c) impregnating the, extrudate with atitanium-containing impregnating agent; and (d) drying and calcining theimpregnated extrudate.
 7. Process as claimed in claim 6, wherein afterstep (b) and prior to step (c) the calcined extrudate is hydrolysed. 8.Process as claimed in claim 6 or 7, wherein the titanium-containingimpregnating agent is a liquid impregnating agent.
 9. Process as claimedin any one of claims 6-8, wherein the titanium-containing impregnatingagent is a gaseous impregnating agent.
 10. Process as claimed in any oneof claims 6-9 which process comprises the additional steps of: (e)hydrolysing the calcined material obtained from step (d), and (f)optionally silylating the product from step (e).
 11. Process for thepreparation of an alkylene oxide by reacting an olefin with an activeoxygen species in the presence of a heterogeneous catalyst as claimed inany one of claims 1-5.
 12. Process as claimed in claim 11, wherein theactive oxygen species is an organic hydroperoxide.